It is heresy against ecclesiastical faith to deny the truth of a canonization.
The object taught by this formula includes all those teachings belonging to the dogmatic or moral area, which are necessary for faithfully keeping and expounding the deposit of faith, even if they have not been proposed by the magisterium of the Church as formally revealed.Such doctrines can be defined solemnly by the Roman pontiff when he speaks ex cathedra or by the College of Bishops gathered in council, or they can be taught infallibly by the ordinary and universal magisterium of the Church as a sententia definitive tenenda. Every believer, therefore, is required to give firm and definitive assent to these truths based on faith in the Holy Spirit’s assistance to the Church’s magisterium, and on the Catholic doctrine of the infallibility of the magisterium in these matters. Whoever denies these truths would be in a position of rejecting a truth of Catholic doctrine and would therefore no longer be in full communion with the Catholic Church.
The truths belonging to this second paragraph can be of various natures, thus giving different qualities to their relationship with revelation. There are truths which are necessarily connected with revelation by virtue of an historical relationship, while other truths evince a logical connection that expresses a stage in the maturation of understanding of revelation which the Church is called to undertake. The fact that these doctrines may not be proposed as formally revealed, insofar as they add to the data of faith elements that are not revealed or which are not yet expressly recopgnized as such in no way dimishes their definitive character which is required at least by their intrinsic connection with revealed truth ...
... With regard to those truths connected to revelation by historical necessity and which are to be held definitively, but are not able to be declared as divinely revealed, the following examples can be given: the legitimacy of the election of the supreme pontiff or of the celebration of an ecumenical council, the canonisations of saints (dogmatic facts), the declaration of Pope Leo Xlll in the apostolic letter Apostolicae Curae on the invalidity of Anglican ordinations... (CDF, Commentary on "Ad Tuendam Fidem")
I know of the contention--but while canonization may lay claim to infallibility--something not all theologians admit--it may not be cause for declarations of heresy. Here are some points to consider.
The pope is not immune from being deceived by false testimony and so his decrees for canonization may be, in fact invalid. I personally believe this is the case with many recent canonizations, particularly that of the much disputed founder of Opus Dei. Such canonizations would only be valid insofar as the testimony provided in the canonization process themselves were actually true; but if this was not the case, then the Pope's act does not make that person a saint.
Most theologians believe what St. Thomas says: "Since the honour we pay the saints is in a certain sense a profession of faith, i.e., a belief in the glory of the Saints [quâ sanctorum gloriam credimus] we must piously believe that in this matter also the judgment of the Church is not liable to error."
But there is a caveat to this mentioned by the Catholic Encyclopedia:
"According to the holy doctor, [this infallibility] is only a point of pious belief. Theologians generally agree as to the fact of papal infallibility in this matter of canonization, but disagree as to the quality of certitude due to a papal decree in such matter. In the opinion of some it is of faith (Arriaga, De fide, disp. 9, p. 5, no 27); others hold that to refuse assent to such a judgment of the Holy See would be both impious and rash, as Suarez (De fide, disp. 5 p. 8, no 8); many more (and this is the general view) hold such a pronouncement to be theologically certain, not being of Divine Faith as its purport has not been immediately revealed, nor of ecclesiastical Faith as having thus far not been defined by the Church."
Simply put, this would mean that while canonizations are generally believed to be infallible, the level of credence by which this must be held is neither that of Divine Faith,nor of ecclesiastical faith. As such, it would not be matter for a judgment of heresy which only involves a formal denial of a dogmatic truth pertaining to the deposit of faith.